As one method of detecting a disease such as arteriosclerosis at an early stage of the disease, a method of diagnosing a blood vessel of its function is drawing a recent attention. Patent Document 1 discloses an example of a blood vessel dynamic characteristic measuring apparatus, which is configured to measure a blood flow velocity distribution within a blood vessel, and to calculate a blood shear rate distribution, a blood viscosity distribution and a blood shear stress on the basis of the measured blood flow velocity distribution. This document proposes the use of the calculated blood viscosity distribution or a blood shear stress distribution as one index value for diagnosing the blood vessel of its function. There is also known a method wherein a diameter of a blood vessel of a subject person at rest (rest-time blood vessel diameter) is measured, and a diameter of the blood vessel is measured after releasing of the blood vessel from blood flow obstruction at a portion of the antebrachium maintained for about five minutes, to calculate a blood vessel diameter increase ratio FMD (blood vessel diameter dilatation amount/rest-time blood vessel diameter*100%) on the basis of an amount of dilatation of the blood vessel diameter after releasing of the blood vessel from the blood flow obstruction, and the rest-time blood vessel diameter, for using this blood vessel diameter increase ratio FMD as the index value for diagnosing the blood vessel of its function. It is also known to use a thickness of the blood vessel wall or a stress acting on the blood vessel wall, as the index value for diagnosing the blood vessel of its function.